The present invention relates to acoustic resonators, and more particularly, to resonators that may be used as filters for electronic circuits.
The need to reduce the cost and size of electronic equipment has led to a continuing need for ever-smaller electronic filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Further, many such devices utilize electronic filters that must be tuned to precise frequencies. Filters select those frequency components of electrical signals that lie within a desired frequency range to pass while eliminating or attenuating those frequency components that lie outside the desired frequency range.
One class of electronic filters that has the potential for meeting these needs is constructed from thin film bulk acoustic resonators (FBARs). These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The sandwich structure is preferably suspended in air. A sample configuration of an apparatus 10 having a resonator 12 (for example, an FBAR) is illustrated in FIGS. 1A and 1B. FIG. 1A illustrates a top view of the apparatus 10 while FIG. 1B illustrates a side view of the apparatus 10 along line A—A of FIG. 1A. The resonator 12 is fabricated above a substrate 14. Deposited and etched on the substrate 14 are, in order, a bottom electrode layer 15, piezoelectric layer 17, and a top electrode layer 19. Portions (as indicated by brackets 12) of these layers—15, 17, and 19—that overlap and are fabricated over a cavity 22 constitute the resonator 12. These portions are referred to as a bottom electrode 16, piezoelectric portion 18, and a top electrode 20. In the resonator 12, the bottom electrode 16 and the top electrode 20 sandwiches the PZ portion 18. The electrodes 14 and 20 are conductors while the PZ portion 18 is typically crystal such as Aluminum Nitride (AlN).
When electric field is applied between the metal electrodes 16 and 20, the PZ portion 18 converts some of the electrical energy into mechanical energy in the form of mechanical waves. The mechanical waves propagate in the same direction as the electric field and reflect off of the electrode/air interface.
At a resonant frequency, the resonator 12 acts as an electronic resonator. The resonant frequency is the frequency for which the half wavelength of the mechanical waves propagating in the device is determined by many factors including the total thickness of the resonator 12 for a given phase velocity of the mechanical wave in the material. Since the velocity of the mechanical wave is four orders of magnitude smaller than the velocity of light, the resulting resonator can be quite compact. Resonators for applications in the GHz range may be constructed with physical dimensions on the order of less than 100 microns in lateral extent and a few microns in total thickness. In implementation, for example, the resonator 12 is fabricated using known semiconductor fabrication processes and is combined with electronic components and other resonators to form electronic filters for electrical signals.
The use and the fabrication technologies for various designs of FBARs for electronic filters are known in the art and a number of patents have been granted. For example, U.S. Pat. No. 6,262,637 granted to Paul D. Bradley et al. discloses a duplexer incorporating thin-film bulk acoustic resonators (FBARs). Various methods for fabricating FBARs also have been patented, for example, U.S. Pat. No. 6,060,181 granted to Richard C. Ruby et al. discloses various structures and methods of fabricating resonators, and U.S. Pat. No. 6,239,536 granted to Kenneth M. Lakin discloses method for fabricating enclosed thin-film resonators.
However, the continuing drive to increase the quality and reliability of the FBARs presents challenges requiring even better resonator quality, designs, and methods of fabrication. For example, one such challenge is to eliminate or alleviate susceptibility of the FBARs from damages from electrostatic discharges and voltage spikes from surrounding circuits. Another challenge is to eliminate or alleviate susceptibility of the resonator from frequency drifts due to interaction with its environment such as air or moisture.